A fluid dynamic bearing device is configured to increase the pressure of a lubricating oil filled in a radial bearing gap between: an outer peripheral surface of a shaft member inserted along an inner periphery of a bearing member; and an inner peripheral surface of the bearing member through relative rotation of the bearing member and the shaft member, to thereby relatively rotatably support the shaft member in a non-contact manner with the pressure (dynamic pressure generating action). The fluid dynamic bearing device is suitably used for, for example, a spindle motor for a disk drive, such as an HDD, by virtue of having characteristics of excellent rotational accuracy and excellent quietness.
An oil-impregnated sintered bearing obtained by impregnating an oil into inner pores of a sintered compact is used as the bearing member to be incorporated in the fluid dynamic bearing device in some cases. However, the oil-impregnated sintered bearing has innumerable minute openings on a surface thereof, and hence when the pressure of the lubricating oil filled in the radial bearing gap is increased, there is a risk in that the lubricating oil penetrates into the inner pores from the openings on an inner peripheral surface of the oil-impregnated sintered bearing, and the pressure (oil film strength) of the lubricating oil in the radial bearing gap is reduced. Such phenomenon is called “dynamic pressure absence,” and is considered as a problem to be avoided in the fluid dynamic bearing device using the oil-impregnated sintered bearing.
As a measure for preventing the dynamic pressure absence, a method involving subjecting an inner peripheral surface of a sintered compact to rotation sizing to reduce surface openings has been known (for example, see Patent Literature 1 below).